This invention is directed to a nonelectrolytic method of forming an oxide coating on an aluminum surface.
Many processes are presently available for forming various types of coatings on aluminum surfaces, such as anodizing, including integral color anodizing, plating, chemical conversion coating, painting and the like. These coatings, although designed for long life, require extensive surface pretreatments and are quite expensive.
A simple, inexpensive process for coating aluminum is described in U.S. Pat. No. 3,726,721 Wittrock et al assigned to the present assignee. Kubie in U.S. Pat. No. 2,975,491 describes a process for forming a coating designed as an extrusion lubricant wherein the aluminum surface is first treated with an ammonium-laden alkaline solution containing a fatty acid or equivalent salt or ester thereof, and then baked at about 400.degree. F. to form a coating having unknown properties except for lubrication. Marosi in U.S. Pat. No. 3,849,207 describes and claims a process for treating aluminum and other metals wherein the surface is treated with an alkaline sodium formate solution and then coated with a clean resinous film to form a sepia-colored coating.
It is against this background that the present invention was developed.